MDD is one of the most severe and debilitating illnesses that affects millions of individuals worldwide. Despite considerable advances over the past decades, a clear understanding of the etiology of MDD is still lacking. Accumulating evidence suggests that MDD may arise from impairments in cellular cascades, which lead to aberrant information processing in the circuits that regulate mood, cognition, and neurovegetative functions. Recently, microRNAs (miRNAs) have emerged as an important class of small non-coding RNAs that by binding to 3' UTR of mRNAs, suppress the translation and/or stability of specific mRNA targets. Since miRNAs show a highly regulated expression, they contribute in the development and maintenance of a specific transcriptome and thus have the unique ability to influence physiological and disease phenotypes. Our recent studies show that the expression of a group of miRNAs is altered in PFC of MDD subjects and that they are involved in coping response to stress. In addition, accumulative evidence points to the involvement of miRNAs in neural plasticity. These studies suggest a strong possibility that miRNAs may contribute significantly to the etiopathogenesis of MDD. We hypothesize that subsets of miRNAs and their variants, regulated in a coordinated fashion, will show differential co-expression in MDD brain, which by affecting specific neural/synaptic mRNA targets and cellular pathways, will participate in MDD pathogenesis. In well characterized postmortem brain samples obtained from MDD and control subjects, we propose to: 1) profile miRNA expression by small RNA sequencing, identify novel miRNAs, and analyze differentially expressed miRNAs; 2) profile mRNA expression by RNA sequencing and identify regulatory relationships between mRNA and miRNA by mapping co-expression network modules; 3) analyze miRNA-mRNA pairs, validate altered miRNAs and specific target genes experimentally, and localize these changes at the cellular level; 4) analyze pathway associated with differentially co-expressed modules in MDD; 5) examine miRNA biogenesis by determining pri-/pre-miRNAs, RISC complexes, and components of biogenesis machinery; and 6) examine the role of synaptic miRNAs in MDD pathogenesis by determining miRNA enrichment via small RNA sequencing in synaptosomes, analyzing target genes and co-expression modules of miRNA-mRNA specifically altered in the synaptic fraction, and examining pri-/pre-miRNAs and components of miRNA biogenesis machinery at synaptic level. By using a combination of the state-of-the-art high throughput small RNA and RNA sequencing, analyzing data by novel bioinformatics tools and validation, identifying changes in miRNAs and their targets in specific cell type(s), and examining the role of miRNAs at the synaptic level in brain regions implicated in mood and cognition, our proposed study is uniquely positioned to advance the field of MDD research at the molecular level. These investigations will provide novel avenues for the development of miRNAs as ''molecular tools'' with the potential to generate new molecular-based therapies to treat this devastating disorder.